By that we mean: Sailing vessels, as usual, have right of way over powered craft, but must yield to commercial vessels plying the lake’s shipping lane (here, as elsewhere, “tonnage rules”).

But from there it gets complicated. Rules of the road often are rewritten on the fly on Lake Union, where the mix of “normal” watercraft — ranging from kayaks that fit on the roof of a Subaru to ocean trawlers that barely fit through the largest of the Ballard Locks — is supplemented by a decidedly abnormal auxiliary flotilla. At any given hour, this might include:

• Four people on a small, mobile dock that has been unsecured from a houseboat, eating dinner at a table with fine white linens, over the soothing hum of a small electric trolling motor.

• Some folks from Connecticut in a rental electric boat, searching for the “Sleepless in Seattle” houseboat at well below the proper trolling speed to catch a sockeye (for newbies: this is not currently allowed, but for the record, about 1.1 mph).

• A couple dozen crew shells, filled with people with brawny shoulders, all training to be extras in “The Boys In the Boat.”

• Some hipsters in Casual Friday office attire dinking their way home to their insufferably chic water-view condos on (what else?) stand-up paddle boards equipped with selfie tripods.

• The usual accompaniment of the garishly painted former military amphibious vehicles — much beloved by hapless tourists, much loathed by most everyone else — known as The Ducks.

• Many actual ducks (which really ought to have sought trademark protection back in the Ivar Haglund era) as well as Canada geese and other small lake-pooping beings, bobbing on the faux-Duck wakes.

• On Tuesday summer evenings (speaking of ducks), the delightfully madcap, Farmer’s-Insurance-Commercial-In-The-Making, sailboat-racing tomfoolery known as the Duck Dodge, wherein 50 to 100 local sailing craft weave in, out and through all the above-named collidables in a “loosely organized,” time-honored sailing race of highly spirited (yes, read that both ways) crews dressed in costumes ranging from formal prom attire to “Game of Thrones” battle wear.

• On very special occasions, a couple of guys in sea kayaks who, upon close, below-water-line inspection, seem to be wearing personal flotation devices — and nothing else except a tan.

Add to this a small naval air force of daredevil Kenmore Air pilots, who are responsible for avoiding all above-mentioned riffraff as they make their way up, up, just barely up enough to clear the funky Godzilla-movie utility towers next to the Interstate 5 Ship Canal Bridge.

It is controlled chaos — and uniquely Lake Union, one of the most simultaneously chaotic and calming urban bodies of water anywhere in North America. The lake we all take for granted — a rare, 200-way intersection of recreation, residences, parklands, heavy industry and (yes!) nature — also holds a few surprises.

Painted like an orca, a Kenmore Air floatplane lands behind two kayakers who take little notice. (Alan Berner/The Seattle Times)

ONE OF THESE is that despite its legacy of decidedly mixed uses — and a few stubborn environmental hangnail hot spots — the lake is generally cleaner and more vital these days than it once was. (It’s even safe to swim in, assuming one doesn’t stir up and eat the bottom sediments, a water-quality expert says.) More on that in a moment. First, consider some particulars about the lake you think you know, but maybe don’t:

• Lake Union, like many of our natural features, was carved by the Vashon Glacier during the last ice age, about 12,000 years ago. Once inhabited by the Duwamish people, whose name for it, XáXu7cHoo, roughly sounded out as “Ha-AH-Chu,” it was later called “Tenas Chuck,” or “small water,” in the Chinook trade jargon. White settler Thomas Mercer (now unfortunately forever linked to the word “Mess,” thanks to the wildly unpopular gridlock corridor bearing his name) lived on the south shore and proved somewhat prescient in 1854, renaming it Lake Union, the eventual unifier of Seattle’s other primary water bodies.

• If you think it looks good up close, you should see it from space. If you define it as all the water between the Fremont and University bridges, Lake Union is shaped like an arrowhead pointing directly at the spectacular crystal orbs outside the new Global HQ of Amazon.com.

• The location is critical. The term of affection its boosters like to use — the lake being the shimmering, jewel-like “heart” of Seattle — really is true. Lake Union lies dead-center among a list of Emerald City destinations, including Fremont, Wallingford, Northlake and the University of Washington to the north; Eastlake and (high above) Capitol Hill’s Volunteer Park to the east; Westlake and Queen Anne Hill to the west; and the present yellow-crane thicket of South Lake Union to the south. The view southwest to the city skyline and the Space Needle from Gas Works Park, a reclaimed toxic industrial site on the north shore, is a signature Seattle scene.

• The lake covers about 600 acres — about 300 less than in 1890, before parts were filled in during various industrial experiments in the city’s past, including the construction of Interstate 5. (Both Westlake and Fairview avenues originally were built on pilings, then filled in; the lake once extended about two blocks farther south.) Despite its current controlled state, serving as a central basin for the man-made waterway connecting Lake Washington to Puget Sound, Union is very much a pre-existing natural lake, with a functioning ecosystem that presents its own set of challenges.

At the Center for Wooden Boats in South Lake Union, people can learn to sail and build boats. (Alan Berner/The Seattle Times)

Given the natural tendency of water to follow the well-known Pete Townshend Law of Hydrology — “The (Salish) Sea Refuses No River” — most of the movement of water through Lake Union is east to west, freshwater to salt. Thus, Lake Union is mostly fresh water — a boon to its use as a mooring basin for ships.

Even so, the lake occasionally is infiltrated by a sub-layer of saltwater, which creeps in via the locks and, being heavier than fresh water, lurks near the bottom. (Saltwater occasionally has been caught sneaking through the murky depths all the way into Lake Washington, through the Montlake Cut, says Tim Clark, a limnologist for King County’s Department of Natural Resources and Parks.)

This often occurs in summer, when rivers such as the Cedar, which feed Lake Washington, are at their lowest flows, and the larger Ballard lock is in frequent operation. The saltwater usually flushes out again during the heavier outflows of winter.

But in one recent year, water samples indicated that a lower layer of saltwater stayed in Lake Union for about 18 months, Clark notes. (This had to do with experimental openings and modifications of the locks for salmon-survival studies, as well as other complicated hydrodynamics probably too advanced even for Townshend, let alone the rest of us.)

On Lake Union, you can find the most expensive to the cheapest seat in the house when it comes to craft that float. The 305-foot-long “Mayan Queen IV,” believed to be one of the largest yachts in the world, is owned by a Mexican mining billionaire. Estimated to cost $140 million when built in 2008 in Germany, it has eight guest cabins and 12 crew cabins. (Alan Berner/The Seattle Times)

LAKE UNION’S ORIGINAL occupants must have found the banks of Tenas Chuck prime real estate. Plenty of current Seattleites still do, most living in condos and/or the ubiquitous houseboats found in small clusters around the lake.

The lake continues to serve a vital role in city industry, traffic, recreation — and perhaps more important, identity.

“It’s a great lake,” Clark says. “It’s fantastic for recreation, a beautiful centerpiece of Seattle, a treasure for commercial vessel traffic, and highlights Seattle’s industrial history.”

Lake Union’s south end was the site of a major sawmill operated by settler David Denny beginning in 1882 — the first of a series of shingle- and sawmills ringing the lake. Early seaplanes built by some guy named Boeing were manufactured along its banks, the first taking flight in 1916. Ford built cars near the south shoreline. The old gas works, along with the former steam electrical plant (1914) now occupied by ZymoGenetics, stand as testament to other past industrial use.

Rigger Jordan St. John helps organize the more-than-800-foot-long anchor chain from a Coast Guard vessel that was in for repairs at Lake Union Drydock. The 98-year-old shipyard and drydock covers more than 12 acres along the lake. (Alan Berner/The Seattle Times)

Shipbuilding and marine outfitting have always been Lake Union staples, and remain so today. Lake Union, especially if the Ship Canal and Salmon Bay near Ballard are considered, is home base for much of the Northern Pacific fishing and crabbing fleet.

Lake Union thus ranks as one of the country’s premier combined working and playing waterfronts. Navigating its shores on a 6-mile loop — designated as a hiking/cycling route by the city, which calls it the Cheshiahud Lake Union Loop, after a local chief — reveals a unique mix of industrial, residential and recreational uses, mostly in hodgepodge fashion.

That random mix of work/play stations can make it daunting for residents, especially newcomers, to get out on the water, notes lake tour operator Geoff Gamsby. His 6-year-old business, Lake Union Charters and Adventures, works hard to change that, hosting Duck Dodge-watching tours aboard the 63-foot 1926 schooner Lavengro, and offering regular sailing classes.

One of the biggest surprises for many first-time passengers is how peaceful the frenetic lake becomes in its “magic” hours after sunset, Gamsby says.

“It’s like being in a jewel box with the lights all round you. It’s always a surprise to our guests how it is to be in such a tranquil place in the heart of a bustling city.”

It’s a white-tableclothed dinner for four organized by Ann Prezyna, second from left, aboard the “M.V. Chip,” as in “chip off the old dock.” Capable of 2 knots, the vessel is powered by an electric trolling motor and must yield to sailboats. (Alan Berner/The Seattle Times)

Other tour companies, including Argosy Cruises, also put people in boat seats on Lake Union. But the average Seattle resident is more likely to dip a toe into Lake Union via what most drive-by observers might consider a surprising number of public access points.

Gas Works Park, situated on the north shore, is the most popular, thanks to its location along the heavily used Burke-Gilman Trail. But the south end has seen a jump in shoreline visits thanks to the dual boosts of the relocation of the Museum of History & Industry to the old Naval Reserve Center building, and the development of Lake Union Park. The nearby Center for Wooden Boats also puts many people on the water via rentals and educational cruises.

Lesser-known, and lesser-accessible, public portals to the lake are found all around its perimeter. Many of these are street-end city parks known as “waterways.” A day spent visiting these sites on foot or by cycle can be a frustrating, traffic-dodging experience. But most of the spaces are pleasant once accessed, and they offer the best up-close look — or even a dip into — modern Lake Union.

One of these, Waterway 15 — due west of Ivar’s Salmon House in Northlake, as the French-fry-bloated seagull flies — contains paving stones and photographs embedded in a walkway that tell the story of the lake’s natural and man-made history. A recent visit there on a sterling July afternoon revealed a “crowd” consisting of two young guys occupying a park bench, smoking a joint, and a female mallard splashing happily on a short stretch of gravel beach.

Another waterway, immediately adjacent to Gas Works, contains what interpretive signs proclaim as the last “natural” beachfront remaining on the lake. It’s a mix of deciduous trees, tucked into an alcove next to Gas Works Park Marina, which is occupied by clusters of diminutive houseboats and a giant construction-crane barge. On a recent visit, while dozens of fresh-air lovers relaxed on the grassy knoll (aka Kite Hill) at the nearby popular park, the only sign of humans at this short stretch of unkempt beach was a pair of discarded bluejeans, lying on a stump at water’s edge.

“LAKE UNION REPRESENTS everything about Seattle, in this one little puddle,” proclaims Dick Wagner, founder of the Center for Wooden Boats, in a video for the lake’s online “virtual museum.” He’s right; for better or worse.

Even those of us who’ve dined upon, swum in, flown over or boated along the lake for decades take for granted its current, relatively clean state — or spend little time worrying about challenges the city’s showcase waterway still faces in terms of making it even cleaner, or at least maintaining the current water quality amid breakneck urban growth.

Polluted-sediment hot spots, at the site of former heavy industry, include the Gas Works area, and other industrial sites in long-industrialized Salmon Bay. Work to clean up the gunk either by dredging or capping continues at a slow pace. The polluted sediments aren’t an immediate danger to humans, but they already have worked their way up into the lake’s food chain.

Overall, the lake’s water quality is relatively good, thanks to steps taken nearly 50 years ago to clean up sewage pollution both here and in Lake Washington. (Lake Union became a literal “cesspool” after the great Seattle Fire of 1889, according to a National Oceanic and Atmospheric Administration history.)

Reserved seats at Gas Works Park will provide a front-row view of the fireworks on the Fourth of July. (Alan Berner/The Seattle Times)

Lake Washington’s primary feeder stream, the Cedar River, already contains relatively clean water because much of its headwaters in the Cascade foothills is protected as a city water source.

A handful of monitoring stations in Lake Union has documented the successful results of secondary sewage treatment and other advances.

“Lake Union’s water quality, in terms of bacteria, phosphorous levels and other factors, has improved over time,” King County’s Clark says. The next step: Current studies are identifying “combined sewer overflow,” or CSO, sources, where contaminants still get flushed into local waterways during storms. A little knowledge — and action — on those sources will go a long way toward keeping the lake as Seattle’s urban waterway “jewel.”

Interestingly, much of the chaotically rapid transformation of the South Lake Union area is good news for the longterm health of the lake, ecologists note. State law since 1988 has required redeveloped parcels to be cleaned up. A decade ago, the southern portion of Lake Union’s drainage was home to 234 building sites contaminated by hydrocarbons, heavy metals and other substances; 122 of these now meet state regulations for ground cleanup, and another 91 were in the process as of July, says Larry Altose, a spokesman for Washington’s Department of Ecology.

“Naturally, these cleanups are good for the lake,” he says. And future uses — keeping the world safe from driving to Safeway and whatnot — are less likely to pollute the groundwater.

All of which is good for the Ducks, big D; the ducks, little d; and everyone else who loves or lives on Seattle’s urban-jewel waterway. When a local lake comes to be part of a civic soul, cleanliness is a highest and best use.

Ron Judd is a Pacific NW magazine staff writer. Reach him at rjudd@seattletimes.com or 206-464-8280. On Twitter: @roncjudd. Alan Berner is a Seattle Times staff photographer.